Passive RFID tags are known, and generally include a resonant tuned antenna coil electrically connected to an integrated circuit (IC). Examples of such RFID tags include: U.S. Pat. No. 5,517,194 (Carroll et al); U.S. Pat. No. 4,546,241 (Walton); U.S. Pat. No. 5,550,536 (Flaxel); and U.S. Pat. No. 5,153,583 (Murdoch).
Systems that employ RFID typically include an interrogator that generates a magnetic field at the resonant frequency of the tuned antenna coil. When the coil is located within the magnetic field, the two couple and a voltage is generated in the coil. The voltage in the coil is magnified by the coil's Q factor and provides electrical power to the IC. With this power, the IC is thereby able to generate a coded identification signal that is ultimately transmitted to the interrogator.
Limitations arise because the resonant current that flows in the tuned antenna coil also generates a magnetic field in the region of the coil. That is, if there is an object—such as a second tag with a second coil—disposed near the first coil, the voltage generated by the first coil (and the second coil as well) will be reduced by the partial cancellation—or even complete cancellation—of these respective fields. In turn, this consequential reduction in power will not allow the first tag (and likely the second tag as well) to reliably provide an identification signal to the interrogator.
In this light, many fields that employ such tags—such as baggage handling services, letter carrying services, inventory management systems, etc.—cannot be processed in “dense” configurations. In other words, such articles must be sufficiently spread apart for the tags—and systems incorporating such tags—to operate reliably. Such “density” limitations thus tend to result in speed and efficiency restrictions.
The discussion of the prior art within this specification is to assist the addressee understand the invention and is not an admission of the extent of the common general knowledge in the field of the invention.